1. Field of the Invention
The present invention relates to a process for solidifying non-settling waste "slime" suspensions, such as encountered in mineralogical operations, to a solid, stable matrix. More particularly, the present invention relates to consolidating finely divided colloidal earthy material in aqueous suspension, such as the slimes derived from phosphate operations for recovery or disposal as solid matter.
Slime ponds develop during many mineralogical operations. The ponds generally comprise a suspension of finely divided earthy materials at a solids concentration of about 0.5 to 20% or more in essentially a non-settling aqueous media and generally include colloidal argillaceous particles finer than about 150 U.S. standard mesh (and most less than 1 micron). The composition of the slime solids will vary depending upon the particular operation; but the non-settling characteristic, whether from mining activities, well drilling, water treatment or other industrial operations producing clay containing ponds that are difficult to settle, is rather constant. Such will be exemplified hereinafter by reference to the phosphatic clay slimes generated in phosphate or beneficiation operations.
In phosphate ore processing, a common procedure is to convert one or more land areas into lagoons by means of low dikes provided with proper outfalls to discharge effluent water, so as to contain the earthy solids washed from the ore and the solids liberated from the matrix during grinding, dressing and other beneficiation operations. As transferred to the impounding area from the beneficiation plant, the slimes may contain as little as about 1.5 to 5% solids on a dry weight basis. The solids concentration increases during settling; but after a level of about 20-25% solids is reached in the over-all pond, a virtually impervious crust of materials containing about 20% water forms over the surface of the slime pond, preventing further evaporation and settling. This becomes a considerable storage problem for continued operations because about 0.5-1.5 tons of slime suspension are produced per ton of finished phosphate ore. Because of the great water content of these aqueous suspensions, the slime volume exceeds the volume of the matrix mined.
A typical wet process phosphoric acid plant also produces about 1.5 to 1.6 tons of phosphogypsum per ton of rock digested. A common procedure to handle the gypsum, is to stack or pile it, initially using two or more lagoon areas. As one area becomes filled, the gypsum stream is diverted to the other and the first is allowed to drain and dry out sufficiently to support mechanical equipment. The dike on the first is then increased in height, using deposited gypsum as a source of diking material and the output shifted from the second to the altered first area. It has been estimated that there are over 2 billion tons of phosphatic clay slime solids currently being stored in lagoons in Florida; and annually, 10-25 million tons are being added to this figure. Yet to date no practical means exists for completely dewatering and consolidating these aqueous colloidal suspensions to a solid matrix.
2. Description of the Prior Art
Both government and industry have been long concerned with the problems posed by the phosphate slime ponds and have conducted considerable research to find a suitable method for dealing with them. Most of the various methods that have been considered focus only on thickening the slimes to a more concentrated aqueous suspension. The Bureau of Mines Report of Investigations No. 6163 entitled "Chemical and Physical Beneficiation of Florida Phosphate Slimes" published in 1963; No. 6844 entitled "Chemical Processing of Florida Phosphate Rock Slime", published in 1966; No. 8611 entitled "Large Scale Dewatering of Phosphate Claim Waste from Central Florida" published in 1982; and Information Circular 8668 entitled "The Florida Phosphate Slimes Problem", published in 1975; and such industry efforts as U.S. Pat. No. 4,051,027 entitled "Settling Clay-Containing Slimes" illustrate these endeavors.
Attempts to consolidate the slime suspensions to a solid material include U.S. Pat. No. 2,947,418 which proposes dewatering the slimes to 40% solids in very thin layers of slime in the settling basin and removing the settled surface water before successive very thin layers are added in order that each layer may dry out before additional slime suspension is added. This is not practical on a commercial basis. U.S. Pat. No. 3,763,041 observes that when the phosphatic slimes are mixed with sand tailings, the slimes dewater faster; and proposes that the mixtures could possess acceptable bearing strengths approaching that of normal soils. U.S. Pat. No. 3,680,698 proposes that the slimes be compacted with tailings by mixing the slime with a liquid coagulant under shearing agitation to flocculate the suspension allowing settling with continued slow agitation to dewater the slime and then adding the tailings in hopes of forming a porous aggregate. However, none of these proposals to consolidate the slimes appears to have been practiced in the field nor gained any commercial success and the conventional practice of passing the freshly generated slime suspension to retention basins for perpetual storage continues.